1. Field of the Invention
The present invention relates to a positive electrode comprising a film-shaped composite active material, which comprises a conducting polymer serving as a first active material, and an electrochemical active material serving as a second active material which is uniformly dispersed in the shaped of particles in the conducting polymer, with the parts-by-weight ratio of the second active material to the first active material being 3 to 9 parts of the second active material to 7 to 1 part of the first active material when the total of the first active material and the second active material is 10 parts. The present invention also relates to a secondary battery in which the above-mentioned positive electrode is used.
2. Discussion of Background
Recent development of small, thin, and light electric appliances is remarkable, particularly in the field of office automation. In accordance with the development of such small, thin and light electric appliances, a secondary battery with higher performance capable of supporting the appliances is demanded.
Under such circumstances, a lithium secondary battery has been rapidly developed as a battery with high energy density, by which conventional lead acid storage batteries and nickel cadmium storage batteries will be eventually replaced.
Hereinafter, an active material which has been worked so as to be provided with predetermined functions for use in a battery is referred to as an electrode. Of a pair of electrodes used in a battery, an electrode on an electrochemically higher potential side is referred to as a positive electrode, while an electrode on an electrochemically lower potential side is referred to as a negative electrode.
As an active material for use in a positive electrode (hereinafter referred to as the positive electrode active material) for such a lithium secondary battery, transition metal chalcogenides such as TiS.sub.2, MoS.sub.2, CoO.sub.2, V.sub.2 O.sub.5, FeS.sub.2, NbS.sub.2, ZrS.sub.2, VSe.sub.2, and MnO.sub.2 are used. Many secondary batteries using such an inorganic material as an active material have been studied.
When such inorganic materials are used as active materials, it is possible to perform a reversible, electrochemical intercalation of lithium ions into and deintercalation thereof from the structure of these inorganic materials. By utilizing this property of the above inorganic active materials, conventional secondary batteries have been developed.
Generally, lithium secondary batteries using the above-mentioned inorganic materials as positive electrode active materials have high energy density because those positive active materials themselves have high densities. Furthermore, since charging is carried out by the deintercalation of lithium ions from the crystalline structure of the above positive electrode active material, and discharging is carried out by the intercalation of lithium ions into the crystalline structure of the positive electrode active material, the battery has an excellent voltage plateau in a discharge curve of an electrode of the battery or in a discharge curve of the battery. However, the diffusion rate of the cations throughout the active material during the electrode reactions at the charging and discharging of the battery is so small that the voltage thereof quickly drops, and fast charging and heavy load discharging properties are apt to be impaired.
In addition, many of the above-mentioned conventionally employed inorganic active materials are generally poor in workability and have insufficient electroconductivities for use in practice, so that when such inorganic active materials are used in a positive electrode, a binder agent and an electroconductive auxiliary agent for imparting electroconductivity to the inorganic materials are generally added.
Such a binder agent for use with the inorganic active materials is required to satisfy the following conditions: (1) being insoluble in an electrolytic solution; (2) having high melting point, and not uniformly miscible with the inorganic active materials; and (3) being sufficiently finely divided particles for use with the inorganic active materials.
As the materials for such a binder agent, polyolefin polymers such as polyethylene, and Teflon are currently used.
As illustrated in FIG. 1, an inorganic active material (a) is fixed with a polyolefin binder agent (b), and electric collection is carried out by an electroconductive auxiliary agent such as acetylene black (not shown) which is interposed between the particles of the inorganic active material.
The above-mentioned polyolefin binder agent (b) does not have a function as an active material. Therefore, when charging and discharging are repeated, that is, when the intercalation of cations into the crystals of the inorganic active material and the deintercalation thereof from the crystals of the inorganic active material are repeated, the polyolefin binder agent (b) lowers not only the efficiency of the intercalation and deintercalation of the cations, but also the energy density per unit weight or unit volume of the active material.
Recently during the development of lithium secondary batteries using such inorganic materials as positive electrode active materials, conducting polymers have been discovered which can perform an electrode reaction by carrying out reversible doping and undoping of an anion and therefore can be used as a positive electrode active material for a lithium secondary battery.
Examples of such conducting polymers so far reported are polyacetylene (refer to, for example, Japanese Laid-Open Patent Application 56-136489), polypyrrole (refer to, for example, the 25th Battery Symposium, Abstracts, P2561.1989), and polyaniline (refer to, for example, the 50th Convention of Electric Science Association, Abstracts, P2281.1984).
Such conducting polymers have the advantages over conventionally employed inorganic materials that they are light, exhibit high power density, excellent electric collection performance due to the electroconductivity thereof, and high cycle characteristics for a 100% depth of discharge, and are also excellent in workability for the fabrication of an electrode.
However, as the research and development of the conducting polymers have proceeded, several shortcomings have also been discovered. For instance, the volume energy density cannot be sufficiently increased because of the low densities thereof, and since the ions which are doped into or undoped from the conducting polymers are supplied from the electrolytic ions in an electrolytic solution, a larger amount of an electrolytic solution is required in comparison with the case where a lithium-intercalation type positive electrode is employed, so that the obtained energy density per unit volume thereof is unexpectedly insufficient for practical use in a battery system.
In order to solve these problems, there is proposed, for instance, in Japanese Laid-Open Patent Application 63-102162 a method of making best use of the advantages of the above-mentioned inorganic active materials and conducting polymers by mutually making up for the respective disadvantages thereof, and fabricating a composite electrode comprising a conducting polymer and an inorganic active material.
In the above Japanese Laid-Open Patent Application, the following procedures (1) and (2) for fabricating the composite electrode are proposed:
(1) a powder-like conducting polymer and a powder-like inorganic active material are mixed in a predetermined ratio with the addition of a binder agent thereto to prepare a mixture of these components. This mixture, with the application of pressure thereto, is molded into a composite electrode provided on a collector; and
(2) an electrconductive monomer is chemically or electrochemically polymerized in the presence of a powder-like inorganic active material, so that a composite electrode is fabricated, which comprises a polymer into which the inorganic active material is incorporated.
In the above-mentioned method (1), since the mixture of the conducting polymer and the inorganic active material is also a powder-like mixture, the formed composite electrode is not thoroughly uniform in quality in its entirety. Therefore, it is extremely difficult to obtain a sheet-shaped composite electrode with sufficiently high strength, density and flexibility for use in practice. Furthermore, since a large amount of a binder agent must be added to the mixture of the conducting polymer and the inorganic active material, a composite electrode with a desired energy density cannot be obtained.
In the method (2), the amount of the inorganic active material that can be incorporated into the polymer is limited, so that a sufficiently high volume energy density for use in practice cannot be obtained.
Thus, it is extremely difficult to fabricate a positive electrode with high energy density for a secondary battery by the conventional methods.
Another problem encountered in the course of the development of a lithium secondary battery is the development of a negative electrode. Conventionally, as a material for the negative electrode of a lithium secondary battery, lithium and lithium-aluminum alloys are used. However, lithium has poor charging and discharging cycle characteristics, and has the risk that short-circuits take place because of the formation of a dendrite; and lithium-aluminum alloys have the shortcomings that it is difficult to fabricate a high voltage battery by using any of lithium-aluminum alloys because the potential of any of the lithium-aluminum alloys tends to shift to a higher potential, although the cycle characteristics are fairly good. Lithium-aluminum alloys also have the shortcoming that they lack in flexibility.
Under such circumstances, lithium secondary batteries using carbon materials capable of performing the intercalation and deintercalation of lithium ions in a negative electrode thereof have been actively developed and recently attracted attention. However, the performance of such lithium secondary batteries is not satisfactory for use in practice.